Zero-tolerance policies are beyond idiotic. A kid at my high school was nearly expelled for almost the same thing. He found a tiny plastic gun in his jacket (like the toy soldier one) and a teacher found out.

I highly doubt a golf ball would 'smoosh' like that. They're made of plastic and cork (and other substances I' too lazy to google.) Those things are built to take a lot of abuse without damage. I suspect it's a rubber ball.